We have used rotavirus single stranded (ss) RNA prepared by in vitro transcription from viral cores as well as cloned rotavirus cDNA as probes to investigate the molecular epidemiology of human rotaviruses and to assess the extent of genetic diversity among human and animal rotavirus strains. The studies carried out thus far suggest that: 1) Genetic variation in human rotaviruses is rather common. 2) Rotavirus strains isolated from individuals of the same animal species show a greater extent of homology among themselves than with strains derived from other species. 3) Rotaviruses undergo genetic variataion by either of two mechanisms: a) accumulation of successive sequence changes within specific genes (genetic drift); and b) gene reassortment (genetic shift) that results in the appearance of rotavirus strains with a constellation of genes which are derived from two or more distinct rotaviruses. The relative importance of these two mechanisms (genetic drift or shift) in the generation of new strains is not known. Partial sequence analysis of nonsocomial rotavirus strains recovered from neonates who underwent asymptomatic infection suggests that the rotavirus genome does not have a high rate of spontaneous mutation; on the other hand, rotaviurs strains have been identified that appear to have been derived by gene reassortment.